The first lady underwent a medical procedure today to treat what her office calls a benign kidney condition. Her office says it went well and there were no complications. She's expected to remain at the Walter Reed National Military Medical Center outside Washington, D.C., for the rest of the week. The White House released few other details. NPR's Richard Harris joins us now to talk about the procedure and why it's often used. Hi, Richard.

RICHARD HARRIS, BYLINE: Hey, Ari.

SHAPIRO: The first lady's office said she underwent an embolization in her kidney. What is that exactly?

HARRIS: Embolization is used to block the blood supply to an artery. It's a standard procedure. And what doctors do is they insert a small tube called a catheter usually in a blood vessel in the groin and thread it up into the kidney in this case. Once it's in place, doctors put something up the tube to block the blood flow. It might be gel particles, or it might be a mixture of alcohol or an oily substance, something like that. The idea is to block off the blood supply to a small part of the kidney.

SHAPIRO: Why would doctors need to or want to block off the blood supply to part of the kidney?

HARRIS: Well, we don't know exactly why they did it in the case of the first lady. But Dr. Keith Kowalczyk, a urologist at the MedStar Georgetown University Hospital, tells me that by far the most common reason is to treat a benign tumor called an angiomyolipoma, which is a tumor formed from the tangle of blood vessels, muscle and fat within the kidney.

KEITH KOWALCZYK: When I think of anyone, especially a female in their 40s or 50s, having an embolization, that's the first thing that comes to mind.

HARRIS: He says that's what leads to this kind of procedure, like, 80 or 90 percent of the time.

SHAPIRO: If that kind of tumor is benign, why would doctors want to treat it?

HARRIS: Well, it could be because the first lady was feeling some symptoms such as pain in her side. Or it's possible that doctors have just simply been keeping an eye on it. These are fairly common. And they are usually treated once they grow to be more than 4 centimeters, which is about an inch and a half across.

SHAPIRO: And just to be clear, we don't actually know that that is what the first lady was experiencing. That's just the most common reason for this procedure that the White House says she had.

HARRIS: That's right. It's also entirely possible that there was some other kind of benign growth in her kidney and that started to bleed. That would be another reason to do this procedure. And if there was actual bleeding, that would be done more on an emergency basis. And there are other more rare reasons to perform this procedure as well, such as a bleeding blood vessel or even some sort of trauma. We simply don't know.

SHAPIRO: Do you make anything of the first lady's office saying that she's likely to remain in the hospital for a week?

HARRIS: I put that question to Dr. Kowalczyk and to another expert in this operation, Dr. David Madoff at the Weill Cornell Medical Center in New York City. Both said that it's hard to know. Some people who get this procedure go home the same day, but more complicated cases do stay in the hospital. It could simply be that doctors treating the first lady are being extra cautious considering who their patient is.